Planar-buried-heterostructure (PBH) semiconductor diode lasers incorporate a two-dimensional variation of a semiconductor energy bandgap for confinement of electron-hole recombination and formation of an optical waveguide within an area of minimum energy bandgap. In such a device an overall planarity of the diode laser device surface is beneficial as it simplifies processing as well as facilitating an optoelectronic integration of multiple devices upon a single wafer. Device planarity has typically been maintained either by regrowth of high bandgap epitaxial material within etched grooves or by the application of diffusion-induced disordering of an as-grown heterostructure.
Reverse biased p-n junctions are typically provided by sequential regrowth of epitaxial material of different doping or by multiple diffusions of different dopants from the surface of the device. All known PBH lasers have employed variations of these techniques to form the required two-dimensional energy bandgap variation.
By example, in "Long-wavelength Semiconductor Lasers", Van Nostrand Reinhold Co. New York, at pages 193-204 G. P. Agrawal and N. K. Dutta provide a review of several PBH-type lasers, including a double-channel planar-buried-heterostructure (DCPBH) laser, fabricated by the epitaxial regrowth of one or more layers. The DCPBH laser is an illustrative example of the complex processing required by these and similar structures. In order to fabricate a DCPBH laser device originally planar epitaxial layers must be chemically etched to form two grooves on either side of the desired laser active region. Subsequently, wide energy bandgap material is regrown inside the grooves with a carefully placed n-type current-blocking layer being included. Growth of the current-blocking layer is critical since the layer must not cover the active-region stripe and thus block current flow to the active layer.
As another example, in a journal article entitled "Low threshold planar buried heterostructure lasers fabricated by impurity-induced disordering", Appl. Phys. Lett. 47 (12), 12/15/85, pp. 1239-1241 Thornton et al. report a PBH laser featuring high temperature diffusion-induced disordering. A high-temperature Si diffusion from a sample surface is employed to compositionally disorder a laser active region to form an optical waveguide and a low energy bandgap recombination region. However, this technique has a disadvantage of leaving highly doped material between the disordered active region and the wafer surface. As a result formation of desired reversed-biased current-blocking regions is difficult, requiring multiple high-temperature diffusion steps and resulting in a non-optimal current-blocking configuration.
In U.S. Pat. No. 4,660,208, issued Apr. 21, 1987, Johnston et al. disclose semi-insulating (SI) Fe-doped InP for use as a current blocking layer in regrown buried-heterostructure (BH) lasers. The technique grows or implants a thin Fe-doped SI InP layer and subsequently regrows the actual laser active layer and top p-type cladding inside of a groove and over the SI InP. The resulting structure forces current to flow around the highly resistive SI material and through the laser active region in order to obtain the high degree of current confinement required for laser structures. An alternative discussed reverses the order of fabrication and grows SI InP within grooves etched in a surface.
However, this structure requires regrowth of semiconductor material over non-planar structures. Other disadvantages relate to the difficulty, expense and low yield of semiconductor regrowth processes. Furthermore, the interface between the first growth and the second growth typically has a high concentration of defects. These defects cause non-radiative recombination of electrons and holes if the p-n junction is allowed to contact the interface.
In U.S. Pat. NO. 4,433,417, Feb. 21, 1984 Burnham et al. teach a non-planar laser grown by MOCVD processes on a non-planar substrate. All but one of the resulting structures have little or no current confinement beyond what is obtained by a conventional proton implant into the surface above the active and waveguide layers. Waveguiding of the laser mode is obtained by thickness variations and bends in the active laser medium created by growth over non-planar substrates. None of the structures disclosed employ the highly efficient BH concept for simultaneous current and radiation confinement. The only structure disclosed in this patent having current confinement more sophisticated than a simple proton implant is illustrated in FIG. 4 and discussed at Col. 8, line 61 to Col. 9, line 42. Here the laser is grown over a set of grooves to provide an optical waveguide. A top surface of the wafer, except for the very top of a ridge which will become the laser, is converted to p-type material prior to regrowth. This method is similar to that disclosed above in U.S. Pat. No. 4,660,208 and suffers from similar drawbacks of complicated regrowth technology and low quality regrown interfaces. Furthermore, this technique results in the creation of a non-planar surface after regrowth. In U.S. Pat. No. Re. 31,806, Jan. 15, 1985, Scrifres, et al. discuss various means for controlling the optical modes of multi-stripe laser arrays for high power applications. There is disclosed implantation or diffusion above an active layer of the device as a means to control the refractive index profile across the device by modulating the injected current density. However, Zn-diffused or implanted regions are intended only to provide the lateral index shift required for coupled-stripe laser operation. An n-type cap layer is said to be employed to confine current injection to the Zn-diffused or implanted stripes in order to enhance the refractive index shift. However, none of the structures disclosed employs implantation or diffusion into the active layer as a means to fabricate a BH type laser.
It is thus an object of the invention to provide for the implantation into a prepared substrate to fabricate a BH type laser.
It is a further object of the invention to provide a single semiconductor growth step and subsequent fabrication of a BH laser by implantation of ions into, and eventual disordering of, a laser active layer outside of a desired laser stripe.
It is a still further object of the invention to provide a single semiconductor growth step and subsequent fabrication of a PBH laser by the implantation of two different doping species into a heterostructure to create reversed-biased current-blocking junctions within a plane of a laser active layer.